A drive shaft is a device that transmits the rotating force of an engine or a gear box to driving axles, and is widely used in machinery including transport machines, such as automobiles, ships and aircraft.
A conventional drive shaft is made of metal and is typically produced by separately producing a tube part and a universal joint part of the shaft and press-fitting and welding them into a single shaft. In the related art, drive shafts made from steel or aluminum are widely used. However, the metal drive shaft is problematic in that it is heavy and has a low lateral directional resonance frequency. Thus, when the length of the metal drive shaft is not less than 2 meters, the drive shaft may resonate within a range of a maximum engine rpm and may break due to the low lateral directional resonance frequency thereof. Therefore, two metal drive shafts, each having a short length of about 1 meter, are separately produced and are connected to each other to form a jointed single drive shaft. However, to connect the two short metal drive shafts to each other, a universal joint must be used, resulting in an increase in the weight and operational noise of the drive shaft.
In an effort to solve the problems with the conventional metal drive shafts, a composite drive shaft, made of a fiber-reinforced composite material, has been proposed and used. The fiber-reinforce composite drive shaft has a higher specific rigidity, a higher specific strength, a higher resonance frequency and a higher vibration damping capability, compared to the conventional metal drive shafts, so that a fiber-reinforced composite drive shaft having a length equal to or longer than 2 meters can be produced and used. Further, when a drive shaft is produced using the fiber-reinforced composite material, the drive shaft eliminates the need for the universal joint, so that the drive shaft can be even lighter and generates less noise. Thus, in advanced countries, such fiber-reinforced composite drive shafts are preferably used in special applications, such as in racing cars or aircraft.
The conventional composite drive shaft is produced through the following process. First, a fiber-reinforced composite material is layered on the circumferential surface of a mandrel, which has a circular cross-section and is coated with a release agent on the surface, prior to winding a compression film made of a high polymer, such as polypropylene or polyethylene, on the surface. Thereafter, the mandrel having the composite material thereon is covered with a vacuum bag, made of a high temperature nylon film, and high temperature and high pressure are applied to the interior of the vacuum bag from external sources in the state in which the interior of the vacuum bag is maintained in a vacuum state using a vacuum pump, thus hardening the composite material. When the composite material on the mandrel has been completely hardened, the mandrel is removed from the hardened fiber-reinforced composite material, thus providing a composite drive shaft.
Another conventional method of producing composite drive shafts has been proposed and is disclosed in Korean Patent No. 241232. The method comprises: layering a fiber-reinforced composite material on the circumferential surface of a mandrel coated with a release agent; inserting a heat shrinkable tube, made of a heat shrinkable material selected from the group consisting of cross-linked polyolefin, polyethylene and polypropylene, into the fiber-reinforced composite material layered on the surface of the mandrel; heating the heat shrinkable tube in an oven, thus allowing the resin to be charged in the fiber-reinforced composite material and hardening the fiber-reinforced composite material; and removing the hardened fiber-reinforced composite material from the mandrel, thus providing a composite drive shaft.
Each of the composite drive shafts, produced through the above-mentioned conventional methods, has a tubular shape, which has a constant cross-section from a first end to a second end thereof. Thus, a variety of techniques for connecting respective connection joints (metal yokes) to the opposite ends of a conventional composite drive shaft have been actively studied and developed. The conventional techniques of connecting respective connection joints to the opposite ends of a composite drive shaft are classified into mechanical fastening jointing and adhesive bonding jointing.
To realize mechanical fastening jointing, a composite material is holed and, thereafter, a connection joint is mechanically fastened to the hole in an associated end of the composite material through pinning, bolting or riveting. However, such mechanical fastening jointing is problematic in that the holing process may damage the texture of the composite material of the drive shaft due to the breakage of fibers of the composite material caused by the holing. Further, because the composite material is an anisotropic material, the stress concentration factor in the mechanically fastened part of the composite material may be increased compared to that of an isotropic material. Another problem with the mechanical fastening jointing resides in that the stress concentrated portion in the composite material may be easily fatigued when a load is repeatedly applied thereto, and noise and vibration may be easily generated in the mechanically fastened part due to the asymmetry of the mechanically fastened part.
Compared to the mechanical fastening jointing, adhesive bonding jointing is advantageous in that it can distribute a load over a larger area and eliminates the need to form holes in the composite material, so that it avoids breaking fibers of the composite material. Thus, adhesive bonding jointing allows the adhesively bonded part to efficiently resist a load repeatedly applied thereto and to be less fatigued, and reduces noise and vibration, unlike the mechanical fastening jointing. However, the adhesive bonding jointing requires a process of treating the surface of the bonded product and is limited by temperature, humidity, etc. Further, the bonding strength of the bonded part may be easily changed according to the skill of a worker during a bonding process. Particularly, because a bonding agent used in the adhesive bonding jointing has a high brittleness index, it is almost impossible to adapt the adhesive bonding jointing to a structure, which may be repeatedly loaded or may receive a load or a torque higher than the bonding strength of a bonding agent used in the adhesive bonding jointing.
Thus, in an effort to overcome the problems of the conventional techniques of connecting the connection joints to a composite drive shaft, a variety of techniques have been proposed. For example, to connect respective connection joints (metal yokes) to the opposite ends of a composite drive shaft, Korean Patent No. 432991 proposed a thermal fitting technique, Korean Laid-open Publication No. 2004-0006568 proposed a thermal press-fitting technique, Korean Patent No. 515800 proposed a mechanical press-fitting technique, and Korean Patent No. 526020 proposed a press-fitting and thermal fitting technique using an insert ring. However, the above-mentioned conventional techniques, using the difference in the physical property between two materials, are problematic in that they require complicated processes, in which heating and/or cooling must be executed to realize the connection of the two materials and the application of an external force must be executed to realize mechanical engagement of the two materials.
The inventor of the present invention has discovered that the problems with the conventional techniques are caused by the fact that the shape of each of the conventional composite drive shafts is tubular, and has a constant cross-section from a first end to a second end. Thus, in an effort to overcome the problems of the conventional techniques, the inventor has completed the present invention paying attention to a mold capable of producing a composite drive shaft, which is configured to allow a connection joint to be easily jointed to each end of the shaft.